In circuits, in particular in DC (direct current) circuits, in which high voltages occur in conjunction with high currents, there is a risk of an arc being formed. Arcs may occur, for example, during maintenance work (disconnection of a line through which current flows), in the event of degradation of contacts on screw connectors or plug-in connectors, at poor solder points or inadequately fastened screw connections or in the event of damaged line insulation. An arc which has been produced can usually only be extinguished by drastically reducing the current flowing via the arc.
In photovoltaic installations (referred to as PV installations in abbreviated form below), arcs are a problem which cannot be underestimated on account of the high DC voltage and the high prevailing direct currents and are one of the main causes of fires in PV installations.
Therefore, reliable methods for detecting arcs which have occurred are of great interest. For reasons of safety, an arc which has occurred should be reliably detected in this case. On the other hand, the likelihood of incorrect detection of alleged arcs should be kept as low as possible, in particular if an automatic switch-off system is associated with the detection of arcs, in which system an automatic restart of the PV installation is prevented, as is required in some countries.
Arcs usually emit a broadband AC (alternating current) signal which extends into the high-frequency range. Methods and apparatuses for detecting arcs which are based on detection of such an AC signal in a circuit are widespread and are described, for example, in the document WO 95/25374 A.
One problem with such methods is that high-frequency interference signals can be incorrectly attributed to an arc, thus resulting in incorrect triggering of the apparatus for detecting arcs. Possible sources of interference signals are, for example, high-frequency radio transmitters, arcs which briefly occur in the current collectors of electric trains or trams which travel past in the immediate vicinity of the system being monitored, or electrical or electronic devices having insufficient electromagnetic shielding or interference signal suppression. Arcs in adjacent circuits may also couple into a system under consideration and may incorrectly trigger the apparatus for detecting arcs. A plurality of parallel DC circuits are often provided in PV installations, a PV partial generator and an inverter being arranged in each DC circuit. All high-frequency signals or signal components in a circuit which do not originate from an arc which has occurred in this circuit are referred to below, in summary, as “interference signals” and their sources are referred to as “interference sources” or “interferer”.
In order to increase the reliability with which arcs are detected, the document U.S. Pat. No. 7,633,727 B2 discloses an arc detection system which has two narrowband bandpass filters operating at different frequencies. An arc is determined as having occurred only when a high-frequency signal having signal components in the two different frequency ranges determined by the bandpass filters is observed. If, however, an interference signal has a frequency spectrum of a similar width to the typical frequency spectrum of an arc, this interference signal cannot be distinguished from a high-frequency signal originating from an arc.
The document EP 1 772 936 A2 discloses an apparatus and a method for detecting arcs in an AC circuit, in which a rate of change of the current in the circuit is detected in addition to the high-frequency signal components. If this rate of change indicates processes in the circuit which might fundamentally be triggers of an arc, for example a sudden current rise or fall, and a high-frequency signal indicating an arc is observed at the same time, it is assumed that an arc is actually present. However, in particular in circuits in which very high overall currents occur, for example in the DC circuits of PV installations, processes which trigger arcs cannot necessarily be clearly detected using a rate of change of the overall current.